Contaminants in the form of dust, rust, carbon, water and oil enter the compressed air/gas system through the atmosphere. Condensates generated during cooling get added to these contaminants and create solid [dust, rust, etc], liquid [oil/water sludge] and vapour [oil and water] pollution in the air. All these are harmful and must be removed. Liquid sludge and solids are removed by centrifugal moisture separator with demister pads, which allow only fine oil/water liquid aerosols and vapours to escape with the air. Removal of liquid aerosol is more complex and difficult, and is achieved by the principle of coalescence, ie, forcing the aerosols to merge with each other. This is done passing the air through a glass microfibre medium where liquids are collected by the fibres, through the processes of inertial impaction, interception and diffusion. Once collected, the micro liquid particles adhere to the fibre surface and gradually buildup into bigger coalesced droplets. These droplets move along the fibres till they attain a mass which makes them break away from the fibres. They are driven to the outside of the filter by the air stream, collected at the outer form barrier and drop out from the filter element at the bottom. The outer foam also does not allow the droplets to re entrain into the air stream. Two fiters are generally used in series - prefilter and microfilter, for removal of larger and smaller aerosols, respectively. In modern machines and equipment, presence of oil/water even at very low level is harmful, particularly where the air is in direct contact with the product, eg, air-jet weaving, knitting, etc, use of air as reactant, pneumatic conveying, bag filter operation, gas manufacturing plants, etc. Removal of oil/water is also very important for air used in instrumentation and controls, powder coating/spray painting, sand/grit blasting, high-speed production machines, etc.
